drm/stm: ltdc: fix late dereference check

drm/stm: ltdc: fix late dereference check

In ltdc_crtc_set_crc_source(), struct drm_crtc was dereferenced in a
container_of() before the pointer check. This could cause a kernel panic.

Fix this smatch warning:
drivers/gpu/drm/stm/ltdc.c:1124 ltdc_crtc_set_crc_source() warn: variable dereferenced before check 'crtc' (see line 1119)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/lkml/202212241802.zeLFZCXB-lkp@intel.com/
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/lkml/202212241802.zeLFZCXB-lkp@intel.com/
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Acked-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515123818.93971-1-raphael.gallais-pou@foss.st.com